The present invention relates to in situ mining of metal values, and more particularly, to downhole heat exchangers for cooling the pregnant leaching liquor in the production holes of an in situ minefield.
The subject matter of the present invention is related to the subject matter of the Patent Application Ser. No. 724,548, filed on Sept. 20, 1976, and entitled "In Situ Mining Method and Apparatus". That application is incorporated by reference in the present application.
As noted in the incorporated reference, much contemporary effort is directed to the development of processes in hardware permitting the efficient and economic extraction of metal values from the low grade porphyry ores residing in relatively large, deep-lying deposits, whereby the metal value extraction may be accomplished with minimal environmental impact.
Generally, in situ mining processes require at least two bore holes drilled to the lowermost level of the desired leaching interval in the ore deposit. A packer and lixiviant injector is then affixed to the interior of a first, or injection, hole at the top of the desired leaching interval. Leach liquor is pumped down the injection hole and into the leaching interval to establish a relatively high pressure reservoir of leach liquor in the portion of the injection hole in the leaching interval. A relatively low pressure is established in one or more nearby production holes at portions of those holes lying within the leaching interval. Lixiviant from the injection hole passes through fissures in the ore along a pressure gradient between the injection hole and the production holes. As the lixiviant passes through the ore, metal values are leached. The pregnant leach liquor is pumped to the surface by way of the production holes and processed to recover the leached metal values.
The effective heat transfer coefficient in production holes is severely limited by the surrounding rock formation and the cement used to case the hole (for example, for a 91/2 inch diameter production hole, the effective heat transfer coefficient for the rock formation may be of the order of 1.5 to 0.7 Btu/hr.degree.Fft.sup.2, with the cement casing and other factors reducing this to provide an overall heat transfer coefficient in a range of 0.6 to 0.4 Btu/hr.degree.Fft.sup.2). At these levels, and with the typical production hole flow rates on the order of 120 gpm, the heat exchange between the pregnant leach liquor and the rock formation, as the liquor travels to the surface, is very small and the leach liquor effectively arrives at the surface substantially at the average geothermal temperature of the leaching interval.
As exemplified by the incorporated reference, in-situ mining techniques require considerable chemical processing at surface plants to extract the leached metal values from the pregnant liquor returned by way of the production holes. For example, in the ammoniated lixiviant process disclosed by the incorporated reference, the surface processing requires an ion exchange plant, operating at a near-atmospheric pressure environment for the leach liquor. However, for deep well insitu minefields (on the order of 3,000 feet), the ambient geothermal temperature in the leaching interval is typically on the order of 100.degree. C. Accordingly, in deep well in-situ minefields, the pregnant leach liquor removed from the production holes is also at a temperature of the order of 100.degree. C. At these temperatures and in the relatively near-atmospheric pressure environment of the surface metal value extraction plant, typically-used lixiviants boil off. Consequently, the systems of the prior art require that lixiviant be constantly replaced in the mine system at the injection holes, or, alternatively, a surface cooling plant is needed for reducing the pregnant leach liquor temperature to the temperatures of the order of 40.degree. C. prior to the metal value extraction processing. The cost of either alternative detracts substantially from the many favorable economic factors associated with in situ mining.
Furthermore, it is well known that stainless steel tubing strings are well suited, particularly in terms of convenience and ease of use, for providing the production hole conduit for removing the pregnant leach liquor from the leaching interval. However, such tubing strings are not generally used due to their relatively high cost, particularly in deep mine environments. Tubing strings made of less expensive material such as fiberglass reinforced plastic (FRP) are typically used. While such tubing strings are relatively inconvenient and difficult to handle, the considerable saving offsets the difficulty factor in terms of economic operating conditions for in situ mines using conventional technology.
It is an object of the present invention to provide sufficiently effective downhole heat exchangers for the production holes of in situ minefields to eliminate, or substantially reduce, the requirement for surface heat exchangers prior to ion exchange processing for metal value extraction.
It is a further object of the present invention to provide downhole heat exchangers having relatively high thermal conductivity, metallic tubing strings for the production holes of in-situ minefields wherein the heat exchangers offset the economic disadvantage normally involved in using such tubing strings in the production holes.